Quantification of pore-size spectrums by solute breakthrough curves

Abstract. Breakthrough of conservative tracers may be used to quantify pore-size spectrum and pore-water velocity distributions in a porous medium. In this study, a theory was proposed to calculate pore-water velocity and corresponding pore-size spectrum in porous media, and its application was demonstrated. Miscible displacement tests of chloride were conducted with sand columns (5 cm id and 5 cm length), repacked with washed sand with a particle size of 2–1, 1–0.45, 0.45–0.325 and <0.325 mm in diameter. The resulting breakthrough curves were divided into approximately 20 segments, and for each segment, a concentration of Cl in an out-flowing effluent was used with corresponding effluent volume and travel time to calculate corresponding pore water velocity (v) and pore-radius. Mean v (v_b) calculated for a column was approximated by geometric averaging the calculated v-values for the BTC. To validate the developed model, laboratory measured and approximated values of v_b were compared. The correlation analysis conducted between measured and approximated v_b resulted in a correlation coefficient of r = 0.89 (P < 0.01). The results revealed that size distribution of effective pores could be quite different even in replicates of small sand columns, which are highly similar in particle-size and total porosity.